One-package polyurethane compositions are known in the art. One example is Pyron, U.S. Pat. No. 3,488,302, which discloses a curable one-part polyurethane composition comprising a polyisocyanate prepolymer and pentaerythritol wherein the ratio of pentaerythritol hydroxyls to prepolymer isocyanate groups is greater than 2:1, and preferably greater than 5:1. In our efforts to find a suitable one-part polyurethane adhesive we prepared compositions substantially as disclosed in Pyron and found that upon curing they were "cheesy" and failed to have the mechanical integrity, bonding strength and tensile strength required in an acceptable adhesive.
Our experience with one-package polyurethane adhesives disclosed in Pyron is reflected in the industry's rejection of one-package polyurethane adhesives in favor of two-package polyurethane compositions. Two-package polyurethane adhesives commonly are available as systems having components that are stored separately and are mixed immediately prior to application. These adhesives are used in product assembly, for example, in the assembly of automobile components, using equipment that automatically and accurately mixes the two components to optimize bond strength and other properties. Before a production run is begun the mixing equipment is pre-calibrated to mix the precise amounts of components at a ratio that produces a polyurethane adhesive having optimum properties. During the production run adhesive is continuously produced and applied. If the flow of adhesive is shut off, the mixing chamber and other parts of the mixing equipment must be solvent-flushed to remove the active polyurethane components. After solvent flushing the mixing equipment must be recalibrated to insure that the components are mixed at the correct ratio. A great deal of adhesive is often wasted during the production runs since the mixing machinery continuously prepares adhesive. As much as two-thirds of the adhesive or more can be lost in this manner. The time, labor and equipment required for mixing the two-part adhesive results in the waste of time, effort, and capital.
Accordingly a need exists for a one-package polyurethane composition that can be heat-activated or heat-cured to a structurally solid mass providing bond and tensile strength, heat, chemical and moisture resistance suitable for automotive component assembly.